While there is a lot of research on the ‘age of the first occurrence’ it is much more difficult to find research on the general frequency of occurrences of febrile seizures with age – regardless of whether it was the first or a subsequent seizure. Unfortunately, parents of children with febrile seizures are well aware of the first occurrence and all they simply want to know is … when is the last occurrence likely to be?
In any case there is one very comprehensive study out of Denmark titled “Register-based studies on febrile seizures in Denmark” which leverages a national database of 1.6 million children born between 1977 and 2004 to produce some of the most detailed statistical results to date. For example, below is a graph from the article clearly showing the peak age of first seizure around 16 months and a sharp reduction in first occurrence after 24 months.
Now, unfortunately this like most studies is only concerned with the age of the first seizure. However, out of the US is a study that just looked at 100 consecutive admissions for febrile seizures to a university affiliated tertiary care hospital. Shown below are some of the results of that study showing simply the age at the time of any febrile seizure.
Again, it clearly shows the peak around 18 months and a sharp reduction after 24 months. But this time, because it is admissions regardless of whether it was the first or subsequent seizures, we can begin to calculate the probability of occurrence of febrile seizures after a certain age (or admission to hospital for them at least). For example:
- admission for a seizure after 36 months was only for 4+3+2+1 = 10 patients of the 100, or 10%.
- after 30 months it was 8+4+3+2+1=18 patients of the 100, or 18%.
- after 24 months it was 11+8+4+3+2+1=29 patients of the 100, or 29%.
And finally, our own Seizure Report results confirm these general findings with 32% of reported seizures occurring after the age of 24 months and only 11% after the age of 36 months.
But as other researchers have argued, these numbers are a bit ‘optimistic’ because subsequent seizures are less likely to result in a hospital admission and/or being reported as parents are more able to cope.
Now unfortunately, while most studies suggest that seizures beyond the age of 6 are very rare, they are still known to occur with no other cause than high temperature. So technically they are still ‘just’ febrile seizures. Accordingly, a special term has been created for such seizures, namely; “febrile seizures plus” with the ‘plus’ relating to the fact that they occur beyond the age of 6. Furthermore, a condition called Generalised Epilepsy with Febrile Seizure-plus (GEFS+) has been described. But again, the chances of a febrile seizure beyond the age of 6 is very, very rare and it is more likely attributable to some other cause.
Bottom line, your child possibly only has a 10% chance of a seizure after the age of 36 months.